Color reproduction processes typically involve using color imaging systems to produce colors on various media. These color imaging systems may be used to duplicate a color image from one medium to another medium, e.g., from one printed copy to another or from a display screen to a printed copy. Color reproduction processes are used in various application environments, for example, color proofing applications. In color reproduction processes, rendering colors similarly across different media is desirable. To reproduce colors accurately, many processes use color coordinate systems known as color spaces to characterize the color output of color imaging systems. One commonly-used color space is Commission Internationale de l'Eclairage (CIE) L*a*b* space.
Color spaces can also be used to characterize the color output of one color imaging system relative to other color imaging systems. Characterizing a color imaging system typically involves calculating a color response function for the color imaging system using the coordinates of the color space, e.g., the L*, a*, and b* coordinates of the CIE L* a* b* space.
Color characterization systems often attempt to account for psychophysical and other effects on human color perception. If inadequately addressed, these effects potentially introduce non-uniformities in color characterization across color imaging systems. These non-uniformities may result in differences in color appearance between different color imaging systems and decrease the accuracy of color characterization.
One psychophysical effect on human color perception, known as adaptation, involves the effect of surrounding colors on the subjective appearance of a color. For example, colors appear darker when viewed against relatively light backgrounds. Conversely, dark backgrounds cause colors to appear lighter than they actually are.
Many color imaging systems are used in connection with displaying colors against a nominally white background or white reference, such as paper or other media. Different color imaging systems may involve producing colors on media having different white references. The human eye perceives the different white references as white and other colors produced on the media relative to the respective white references. As a result, a single objective color is subjectively perceived differently against different backgrounds.
Some color characterization systems attempt to compensate for differences in white references by using transformations to modify tristimulus values based on the white reference. While such systems are effective in characterizing relative color within a single color imaging system, given a particular imaging medium and a particular set of viewing conditions such as illumination and viewing angle, many of these systems produce less accurate results when transforming colors between different color imaging systems. One problem inadequately considered by many color characterization systems is the effect of variations in white reference for different color imaging systems due to differences in the imaging substrate or display white point. For example, some color characterization systems that use the CIE L*a*b* color space produce non-uniformities in matching color proofing systems having different imaging substrate white reference. In particular, some such color characterization systems have been observed to produce severe non-uniformities when mapping from a substantially white imaging base to a somewhat blue-shifted imaging base.
Other color characterization systems using different color spaces have been observed to produce uniform mapping in light shades of colors, but less uniform mapping in more intense colors. As a result, after a transformation function has been generated to match color values between two color imaging systems, a human operator typically engages in significant empirical adjustment to obtain an acceptable visual match. This empirical adjustment is potentially labor-intensive and time-consuming.
Another psychophysical perceptual effect is known as black point adaptation. This phenomenon involves the perception of a near-black color as black despite the presence of stray light that imparts non-zero tristimulus values to the near-black color. This effect is particularly noticeable when a color characterization system attempts to characterize perceived colors on a computer monitor, where near-black colors appear black despite significant stray light that devices measure as having tristimulus values significantly higher than zero.
Other color imaging systems are also susceptible to perceptual effects attributable to black point adaptation. For example, using certain color characterization systems to simulate newspaper colors on opaque paper results in reproduced colors that appear washed out. This result is attributable to the significant non-zero tristimulus values for the darkest color or black reference formed on newsprint. These non-zero tristimulus values are partially attributable to stray light scattered from the coarse fibers and partially attributable to inadequate ink coverage of the newsprint. Many conventional color characterization systems fail to adequately compensate for black point adaptation.